1. Field of the Invention
The present invention is directed to a high-frequency safety fuse of the type having a fusible conductor with a cross-sectional area capable of carrying a high-frequency current, and having two electrical terminals respectively connected to opposite ends of the fusible conductor.
2. Description of the Prior Art
A safety fuse of the type described above is disclosed in European Application 0 270 954. This known safety fuse is basically suited for protecting high-frequency circuits. A disadvantage in the use of the fuse for protecting such circuits, however, is that a fuse of this type has a relatively high high-frequency equivalent resistance as a consequence of the fuse having a low cut-off current. If a fuse having a low equivalent resistance is required in addition to the requirement of a lower cut-off current, a fusing characteristic which is slow to react must be accepted as a compromise.
Safety fuses used in the megahertz range are used, for example, in the circuit for local coils in a nuclear magnetic resonance tomography apparatus. Local coils are normally employed only for the reception of nuclear magnetic resonance signals. The nuclear spins are excited by means of a transmission pulse transmitted from another antenna, for example a whole-body antenna. The local coil or coils must be decoupled for the duration of the excitation pulse, i.e., the high-frequency magnetic field of the transmission pulse cannot induce any currents in the local coil or coils. This decoupling is usually achieved by an electronic switch having PIN diodes. If this switch malfunctions, extremely high currents, having an effective value up to 10 A, can be induced in the local coil in the extreme case, so that a risk to the patient exists. The cut-off current of the safety fuse used in the local coil circuit must be small enough to preclude risk to the patient.
When safety fuses are used in high-frequency measurement circuits, they must have a low high-frequency equivalent resistance in addition to having a low cut-off current with an effective value on the order of magnitude of a few hundred mA, so that the signal-to-noise ratio is not degraded. In order to meet both demands, fuses which are slow to react must usually be utilized given applications in the megahertz range. Such fuses, however, have a long fusing time of approximately two minutes, given a cut-off current which amounts to twice the nominal current of the fuse.